The present invention relates to a carbon monoxide oxidation device.
Fuel cells electrochemically produce electricity from reactants supplied externally thereto. Particularly, hydrogen is often used as fuel and is supplied to the fuel cell's anode. Oxygen or air is used as oxidant and is supplied to the fuel cell's cathode. The hydrogen may be stored in a hydrogen storage but for mobile application it has proven useful to produce the hydrogen from liquid hydrocarbon fuels, e.g. from diesel in a fuel reformer. Unfortunately, the hydrogen rich reformat gas exiting the fuel reformer contains significant amounts of carbon monoxide that is toxic to the catalyst used in the fuel cell and must be therefore removed or at least reduced to a very low concentration.
It is known that the carbon monoxide, content of the reformate can be reduced by the so-called “water-gas shift” reaction that usually takes place in one or more separate shift reactors located downstream from the reformer. In the water-gas shift reaction, water (i.e., steam) reacts with the carbon monoxide in the reformate according to the following ideal exothermic shift reaction:CO+H2O→CO2+H2
Additionally, it is known to arrange two water-gas shift reactors in series, wherein the first shift reactor is a high-temperature, adiabatic shift reactor in which the reformate enters the reformer at a first temperature and exits it at a somewhat higher temperature. Thereafter, the reformate is cooled and enters a second shift reactor which is an isothermal, low temperature shift reactor in which the inlet and outlet temperatures of the reformate is essentially the same. These shift reactors usually comprise a housing containing a catalyst bed through which the reformate flows, and to which additional steam may be added or wherein the steam contained in the reformate gas is used.
Additionally or alternatively, it is known to use at least one preferential oxidation reactor (PrOx) in which the carbon monoxide content of hydrogen-rich reformate is selectively reacted with air at a temperature of below 280° C. Thereby, the carbon monoxide concentration in the reformate may be reduced to the desired amount of below 0.00005 mole fraction in order to use the reformate in the fuel cell without poisoning the fuel cell catalyst. In the PrOx reactor, the air preferentially oxidizes the carbon monoxide in the presence of the hydrogen, but without consuming/oxidizing substantial quantities of the hydrogen. The PrOx reaction is exothermic and proceeds as follows:CO+½O2→CO2
When the system reaches steady state, and the carbon monoxide level is low enough, the PrOx reactor effluent is supplied to the fuel cell. Before the carbon monoxide level is low enough, the PrOx effluent is shunted around the fuel cell for temporary use elsewhere in the system.
In all of the above described systems, the efficiency of the carbon monoxide oxidation depends on the homogeneity of the gas mixture of reformat gas and the oxidation agent (i.e. air or steam). Additionally, it is desired to minimize the addition of oxidation agent as excess oxygen or steam reacts with the hydrogen and therefore decreases the hydrogen amount in the hydrogen rich gas useable for the fuel cell.
Additionally, it has been shown that particularly in the known water gas shift reactors the temperature distribution should be as constant as possible over the cross section of the reactor for establishing a maximum efficiency.
From the state of the art, e.g. from US 2006/115394 a carbon monoxide oxidizer is disclosed which comprises a mixing unit that is adapted to mix at least two kind of gases as a mixed gas. Thereby, the mixing unit comprises a stackered body of a plurality of plates, wherein the stackered body comprises a rotating passage provided by through hole formed in each of the plates to rotate a flow of the mixed gas. Additionally, the document discloses an oxidant gas supply unit in form of an orifice plate with a rectangular opening in a center and radially arranged oxidant channels, which swirls an oxidant gas toward the opening. The mixing unit comprising the stackered plates is arranged downstream of the orifice plate, whereby a chamber is defined between the mixing unit and the orifice plate. The mixing unit in turn is arranged in close vicinity to the catalyst.
The disadvantage of the known carbon monoxide oxidizer is that the orifice plate, the chamber and particularly the stackered plates of the mixing unit require a lot of space, whereby the carbon monoxide oxidizer as such becomes very bulky.
It is therefore desirable to provide a carbon monoxide oxidation device requiring less space and providing an optimal mixing of reformat gas and/or oxidizing agent and/or an improved temperature distribution over the cross section of the oxidation device. Additionally, it is desirable to provide a carbon monoxide oxidation device which has a low pressure drop from the inlet to the outlet, whereby the energy consumption of the carbon monoxide oxidation device may be reduced.
A carbon monoxide oxidation device according to an aspect of the present invention is provided.
In the present application a carbon monoxide oxidation device for oxidizing carbon monoxide contained in the hydrogen rich reformat gas is proposed. The below described oxidation device comprises a housing which incorporates an oxidation catalyst for oxidizing the carbon monoxide of the reformat gas by means of an oxidizing agent to carbon dioxide. Upstream of the catalyst the housing comprises at least one gas inlet for providing a gas stream of at least said reformat gas into the housing, and downstream of the catalyst the housing comprises a gas outlet for exiting treated gas from the housing. The housing further incorporates a gas stream perturbation device which is arranged upstream of the catalyst and adapted to provide a perturbation in the gas stream.
Even if indicated as reformate gas it is clear for a person skilled in the art that any fluid may be used, not only gaseous fluids.
In order to provide a carbon monoxide oxidation device having a reasonable installation space, the gas stream perturbation device is designed as at least one propeller-shaped plate with a plate portion having a surface facing the gas stream and at least one blade which is connected to the plate portion and has a leading edge and an effluent edge, wherein a surface defined between the leading edge and the effluent edge is inclined in relation to the surface of the plate portion with a predetermined blade inclination angle, whereby at least one opening in the plate is defined. By the new design of the gas stream perturbation device it is possible to reduce the space demand of the carbon monoxide oxidation device significantly.
Additionally, the described gas stream perturbation device provides an improved mixing of the gas stream flowing through the opening defined by the inclined blade. Further, the inventive gas perturbation device causes only a low pressure drop, whereby energy necessary for the operation of the carbon monoxide device, e.g. for operation of an air compressor may be reduced, which in turn renders the carbon monoxide oxidation device more cost effective.
According to a preferred embodiment the at least one blade is inclined in circumferential direction. By the inclination in circumferential direction, a gas stream streaming onto the gas stream perturbation device is guided through the opening and enters a space downstream of the gas stream perturbation device in a tangential direction. Thereby, a swirling motion of the gas stream is introduced, which provides a homogenous mixture of the gas stream downstream of the gas stream perturbation device.
It should be noted that in general a mixture may be a mixture of at least two different types of fluids, but “mixture” may also be understood in that way that the complete gas streaming through the reactor is mixed through, whereby the homogeneity of the gas stream may be increased and a more even temperature distribution may be provided.
According to a further preferred embodiment the opening is defined between the leading edge and the effluent edge of the blade. Thereby, a preferably substantially axial aligned opening is defined which provides the desired perturbation downstream of the perturbation device. Additionally, it is preferred if the leading edge and the effluent edge are axially aligned or overlap each other so that the desired perturbation is achieved.
Further, it is preferred if the gas stream perturbation device is stationary mounted inside the housing with its outer circumferential rim contacting and being fluidtightly sealed to an inner wall of the housing. Thereby, the gas which reaches the gas stream perturbation device is forced to stream through the at least one opening, whereby the above described perturbation is provided.
According to a further preferred embodiment the plate portion of the gas stream perturbation device is centrally arranged in the housing and the at least one blade is arranged between the centrally arranged plate portion and an inner wall of the housing. By arranging the openings in an outer circumferential portion of the oxidation device, a particularly well and homogenous mixing may be achieved.
According to a further preferred embodiment the gas stream perturbation device comprises at least two, preferably four equally spaced blades correspondingly defining two, respectively four openings, wherein the openings are arranged between the leading edge and the effluent edge of adjacent blades. This design preferably ensures that a pressure drop may be kept low which in turn allows for a reduced compressor power. This in turn increases the overall efficiency of the whole system.
According to a further preferred embodiment the carbon monoxide oxidation device has a substantially circular or elliptical cross section and the gas stream perturbation device is substantially disk-shaped. A cylindrically shaped housing is preferred as its circular or elliptical cross section supports the circular perturbation motion of the gas downstream of the gas stream perturbation device.
According to a further preferred embodiment the gas stream perturbation device comprises a first and a second propeller-shaped plate, wherein the first and second openings provided by the inclined plates are misaligned. The double plate arrangement allows for a homogenous mixing as well as for a design of the reformer where an oxidizing agent inlet is not provided upstream of the gas stream perturbation device but is arranged between the first and the second plate. Thereby, the first plate may provide a perturbing motion of the gas stream into which the oxidation agent is introduced. Thereby, the oxidation agent is already premixed with the reformate gas. The second plate arranged downstream of the first plate and of the oxidizing agent inlet in turn even improves the mixing of oxidizing agent and of the reformate gas.
Even if it is preferred to arrange the oxidation agent inlet upstream of the gas stream perturbation device but downstream of the gas inlet it is also possible to arrange the oxidizing agent upstream of the gas inlet whereby a mixture of reformat gas and oxidizing agent is introduced into the housing.
According to a further preferred embodiment the gas stream perturbation device is arranged in a predetermined distance from the catalyst, whereby a perturbation development chamber for the gas stream is provided. Thereby, it is preferred if the gas stream perturbation device is arranged in a distance D from the catalyst, wherein a ratio of the distance D to a length L of the housing of the carbon monoxide oxidation device is greater as or equal to 0,2:D/L≥0,2.This arrangement allows a sufficiently large perturbation development chamber without increasing the overall length of the carbon monoxide oxidation device.
Further, it should be noted that a size of the at least one opening defined by the inclination angel of the at least one inclined blade predetermined blade angle is calibrated to the expected gas flow and/or the volume of the distribution development chamber and/or the total number of blades and/or the diameter of the housing and/or the length of the housing. Thereby, it is preferred that a volume of the perturbation development chamber, particularly defined by the distance between the gas stream perturbation device and the catalyst is calibrated to the expected gas flow and/or a diameter of the housing and/or a length of the housing.
The carbon monoxide device may be designed as high-temperature WGS reactor, low-temperature WGS reactor, or as PrOx reactor. The new design of the gas stream perturbation device increase the efficiency in all carbon monoxide oxidation systems.
Exemplarily, the carbon monoxide oxidation device may be a high temperature water gas shift reactor operating at a temperature of above 280° C., comprising a noble catalyst, e.g. platinum of a platinum containing composition, or a non-noble metal catalyst, such as a mixture of Fe3O4, Cr2O3, and MgO. Steam is used as oxidizing agent, which may be introduced into the housing by a separate steam inlet upstream of at least one propeller-shaped plate of the gas stream perturbation device. Alternatively, no steam is added to the reformate gas, but the steam provided during the fuel reforming and still contained in the reformate gas may be used. In case no steam is added, the gas stream perturbation device acts mainly as temperature distribution device which provides a homogenous and even temperature distribution.
Steam from the reformate or as addition may also be used for a low temperature water gas shift reactor, which operates at a temperature of below 280° C. Also, the low temperature water gas shift reactor comprises a noble or non-noble catalyst, but in a different composition than the high temperature water gas shift reactor.
According to a further embodiment, the carbon monoxide device is a preferential oxidation reactor PrOx operating at a temperature of below 280° C. The PrOx comprises a heterogeneous catalyst. The catalysts may include noble metals such as platinum, platinum/iron, platinum/ruthenium, gold nanoparticles as well as copper oxide/ceramic conglomerate catalysts. In contrast to the water gas shift reactor oxygen or air is used as oxidizing agent, which is introduced into the housing by a separate oxygen/air inlet upstream of at least one propeller-shaped plate of the gas stream perturbation device. Additionally or alternatively, reformat gas and oxygen/air may also be introduced as pre-mixture into the housing of the PrOx through the gas inlet.
Further preferred embodiments are defined in the description, the drawings and the attached claims.
In the following similar or similarly functioning elements are indicated with the same reference signs.